This invention relates to the cubic form of boron nitride and its formation or transformation from the hexagonal form of boron nitride. More particularly, this invention relates to utilizing large HBN particles of an ideal structure in forming cubic boron nitride. The processes of this invention include the subjection of the hexagonal form of boron nitride to high pressures and temperatures, both in the absence and presence of a catalyst to form cubic boron nitride.
Three crystalline forms of boron nitride are known: (1) hexagonal boron nitride (HBN), a soft graphitic form similar in structure to graphite carbon; (2) wurtzitic boron nitride (WBN), a hard hexagonal form similar to hexagonal diamond; and (3) cubic boron nitride (CBN), a hard zincblend form similar to cubic diamond. The three boron nitride crystal structures may be visualized as formed by the stacking of a series of sheets or layers of atoms. FIGS. 1-a through 1-c of U.S. Pat. No. 4,188,194 illustrate these three structures in greater detail. In HBN crystals, the boron and nitride atoms bonded together are in the same plane as stacked layers. In the more dense CBN crystal structures, the atoms of the stacked layers are puckered out of plane. In addition, the layers are stacked along the [001] direction in HBN crystals, whereas in the CBN crystal, the layers are stacked along the [111] direction. Furthermore, bonding between the atoms within the layers of an HBN crystal is predominantly of the strong covalent type, with only weak Van derWaals bonding between layers. In CBN crystals, strong, predominantly covalent tetrahedral bonds are formed between each atom and its four neighbors.
It is the cubic form of boron nitride which finds use as an abrasive material typically in the form of a cluster compact, a composite compact, or various types of grinding wheels. A cluster compact is defined as a cluster of abrasive crystals bonded together either in (a) a self-bonded relationship, (b) by means of a bonding medium or (c) by some combination of the two. U.S. Patent Nos. 3,136,615 and 3,233,988 provide a detailed description of certain types of cluster compacts and methods for their manufacture.
A composite compact is defined as a cluster compact bonded to a substrate material, such as a cemented tungsten carbide. The bond to the substrate can be formed either during or subsequent to the formation of the cluster compact. U.S. Pat. Nos. 3,743,489 and 3,767,371 provide a detailed disclosure of certain types of composite compacts and methods for their manufacture.
Cluster compacts and composite CBN compacts are a tough, coherent, high-strength mass of a plurality of chemically bonded CBN crystals used in machine dressing and drilling.
Cubic boron nitride particles are also used as aggregates physically bonded together by a metal matrix such as nickel. A grinding wheel is one example of such an aggregate.
Methods for converting HBN into CBN monocrystalline and polycrystalline particles are well known. U.S. Pat. No. 2,947,617 describes a method for preparing cubic boron nitride by the subjection of a hexagonal form of boron nitride, in the presence of a specific additive material, to very high pressures and temperatures. The pressures and temperatures are within the cubic boron nitride stable region defined by the phase diagram of boron nitride. Cubic boron nitride is recovered after removal of the high-pressure and high-temperature condition. The added material or catalyst is selected from the class of alkali metals, alkaline earth metals, tin, lead, antimony and nitrides of these metals. The cubic boron nitride stable region is that represented in FIG. 1 of U.S. Pat. No. 2,947,617 shown above the equilibrium line on the phase diagram therein.
A method for conversion of HBN to CBN in the absence of catalysts is described in U.S. Pat. No. 3,212,852 under conditions of higher pressures and temperatures. See also: Wakatsuki et al., "Synthesis of Polycrystalline Cubic BN (VI)," and Ichinose et al., "Synthesis of Polycrystalline Cubic BN (V)," both in Proceedings of the Fourth International Conference of High Pressure, Kyoto, Japan (1974), pp. 436-445; U.S. Pat. No. 4,016,244; Wakatsuki et al., Japanese Patent No. Sho 49-27518; Wakatsuki et al., Japanese Patent No. Sho 49-30357; Wakatsuki et al., Japanese Patent No. Sho 49-22925; Wakatsuki et al., U.S. Pat. No. 3,852,078; Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride," Mat. Res. Bull. 7, 999-1004 (1972); and Sirota, N. British Patent No. 1,317,716. Such methods are referred to as direct conversion processes. HBN can be directly converted to CBN compacts in the absence of catalysts according to U.S. Pat. No. 4,188,194. U.S. Pat. No. 3,918,219 teaches catalytic conversion of HBN to CBN in contact with a carbide mass to from a CBN composite body.
In all of these processes, hexagonal boron nitride is used as a starting material. Two forms of hexagonal boron nitride have been identified, the turbostratic structure and the ideal structure. The turbostratic structure is characteristic of pyrolytic boron nitride (PBN) which has continuous two-dimensional layers of hexagonal rings stacked at irregular intervals and randomly oriented. The ideal structure is characteristic of graphitic boron nitride (GBN) wherein the boron and nitride atoms alternate in an orderly and continuous fashion in the stacked sheets of 6 membered rings.
PBN is a low-pressure form of HBN, made typically by chemical vapor deposition of BCl.sub.3 +NH.sub.3 vapors on a graphite substrate. As deposited, it has a high purity of 99 99+%, a density of about 2.0-2.18 g/cm.sup.3 and a preferred orientation of the layer plates between 50/ and 100/ in the [001] direction. The ideal structure HBN (GBN) has a higher density of 2.28 g/cm.sup.3. The interlayer spacing in the pyrolytic materials is also greater than that of the ideal structure HBN (typically 3.42 for PBN compared to 3.33 for GBN).
Both PBN and ideal structure HBN are used as powders for conversion to CBN. Although PBN is available in the form of sheets, it is milled to powder form, which is typically high-aspect ratio plate-like particles which can be sieved to a particular mesh size. The use of large particle PBN has been found to reduce the packing density within the cell of the high pressure equipment used to convert to CBN, which may be undesirable.
Hexagonal boron nitride of the ideal structure has been available as a powder with an average particle size of less than 10 microns, typically 5-6 microns, presumably due to the nature of its manufacture.
In that the ideal structure HBN is provided in the form of particulates, the cell used in high pressure/high temperature equipment cannot be completely packed with material due to spaces between the particles, as with PBN. The density of press-pills obtained from particulates of ideal structure HBN are far below the theoretical maximum of 2.28 gm/cc, typically about 1.85 gm/cc.
In preparing cubic boron nitride from ideal structure hexagonal boron nitride, it is desirable to maximize the density of the HBN starting material in the reaction vessel to maximize the yield from an operating cycle of the high-pressure, high-temperature press and reduce the press-stroke in the equipment utilized.